universal-access: Don't use widget names for GSettings keys

The widget were renamed in 6cd94618 and this setting was broken.

Fixes #555
11 jobs for zoom-setting in 4 minutes and 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #317493
build

00:01:36

 
  Test
passed #317495
asan

00:02:26

passed #317494
test

00:00:44

 
  Manual
manual #317500
allowed to fail manual
armv8
manual #317496
allowed to fail manual
flatpak
manual #317499
allowed to fail manual
msan
manual #317501
allowed to fail manual
ppc64le
manual #317497
allowed to fail manual
tsan
manual #317498
allowed to fail manual
ubsan
 
  Review
manual #317502
allowed to fail manual
review
manual #317503
allowed to fail manual
stop_review